The LG L3 (MSM7225A) shipped with kernel 2.6.38. Moving it to 3.4 for Android 4.2.2 was a chain of silent reboots, nearly all caused by the modem.
I enabled the 2.6.35 kernel's USB host, forced host mode and measured the PHY: with a flash drive or mouse, D+ and D- never move.
The LG L3 on kernel 3.4 reboots by itself, no panic, at 609–616 s of uptime; on 2.6.38 it doesn't. What I ruled out and what's still open.
The Y220-U05 (MT6572) runs a kernel from a donor tree. What had to be fixed: boot, Wi-Fi, camera, power-off charging and the hotspot.
Before booting the Y220-U05's Android with my own kernel I tried each version as a recovery. How I built that test bench and what it taught me.